The Rosetta Instrument Scientists Used to Reveal the Comet’s Water-Ice Cycle

It’s called the Visible, InfraRed and Thermal Imaging Spectrometer, or VIRTIS. And it’s one of 11 science instrument packages aboard the ESA’s Rosetta probe as it orbits Comet 67P/Churyumov-Gerasimenko.

What is VIRTIS? It’s an imaging spectrometer that takes three data channels and combines them into one instrument. Two of the data channels are dedicated to spectral mapping while the third is used for spectroscopy (studies the interaction between matter and electromagnetic radiation).

– To study the cometary nucleus and its environment– Determine the nature of the solids of the nucleus surface– Identify the gaseous species– Characterise the physical conditions of the coma– Measure the temperature of the nucleus

This week, VIRITS revealed evidence of a daily water-ice cycle on the surface of comments.

“We found a mechanism that replenishes the surface of the comet with fresh ice at every rotation: this keeps the comet ‘alive’,” says Maria Cristina De Sanctis from INAF-IAPS in Rome, Italy. She’s also the lead author of the study.

Since Rosetta arrived alongside Comet 67P in August 2014, scientists have been busy investigating processes on the comet’s surface. In particular, the outgassing that occurs. You can see this outgassing in the image below when Comet 67P was at perihelion (closest point to the sun).

Maria Cristina and her team poured over a data set from September 2014. The scientists focused on a one square km region located on the comet’s ‘neck.’ This was one of the most active areas on the comet at the time. The team noticed changes in illumination in the area during the 12 hours it takes for the comet to complete a full revolution.

“We saw the tell-tale signature of water ice in the spectra of the study region but only when certain portions were cast in shadow,” says Maria Cristina.

“Conversely, when the Sun was shining on these regions, the ice was gone. This indicates a cyclical behaviour of water ice during each comet rotation,” Maria Cristina added.

What’s happening is that water ice on and just below the surface sublimates as sunlight hits it. It’s turning into a gas and flowing away from the comet just like you see in the image above. As the comet continues to rotate, it plunges the region into darkness. The surface rapidly cools, and some of the water vapor recondenses on the cold surface.

“By this mechanism, the surface layer becomes enriched in water ice,” the scientists wrote. “The water ice in the uppermost surface layers will be stable until a new cycle of solar illumination starts, which will increase the surface temperature and thus trigger again the outgassing of water from the comet.”

Here’s the most recent look at Comet 67P.

Rosetta was 330 kilometers away when it snapped the image on September 21st using its NavCam.